The present invention relates to a method for improving the bioavailability of polysaccharides in lignocellulosic materials, involving reacting lignocellulosic materials with ammonia and ethanol.
Interest in renewable liquid fuels such as ethanol is increasing worldwide. In the United States, the goal is to produce 60 billion gallons of ethanol per year to replace 30% of the nation's gasoline consumption by 2030 (U.S. Department of Energy, Breaking the Biological Barriers to Cellulosic Ethanol: A Joint Research Agenda, Report No. DOE/SC-0095 (2006)). Currently most of the fuel ethanol produced commercially in the U.S. is made from corn. Corn ethanol alone is not sufficient to meet the stated goal since the maximum production has been estimated to be only about 12 to 15 billion gallons per year (Collins, K., The New World of Biofuels: Implications for Agriculture and Energy, EIA Energy Outlook, Modeling, and Data Conference, Mar. 28, 2007, Washington, D.C.; Hess, R., et al., Cellulosic Biomass Feedstocks and Logistics for ETOH, GEC Meeting, Feb. 27-28, 2007, Washington, D.C.). Thus, a renewable feedstock for ethanol production other than corn is needed, and considerable attention has been given to lignocellulosic biomass. The three main sources of lignocellulosic biomass are forest products and residues, agricultural residues, and dedicated energy crops. Corn stover contributes the largest quantities among the agricultural residues in the U.S. Currently 75 million metric tons (MT) of corn stover can be sustainably collected and used for ethanol production. It has been estimated that by the mid-21st century, 232 million dry metric tons of corn stover would be available for ethanol production (Perlack, R. D., et al., Biomass Feedstock for a Bioenergy and Bioproducts Industry The Technical Feasibility of a Billion-ton Annual Supply, U.S. Department of Energy and U.S. Department of Agriculture (2005)). The potential ethanol yield of corn stover has been estimated to be 0.29 liter per kg (76.61 gallons per MT) (Kim, S., and B. E. Dale, Biomass and Bioenergy, 26: 361-375 (2004)). Thus, by the mid-21st century, 18 billion gallons of ethanol can be produced from corn stover alone.
Lignocellulosic feedstocks consist of three main components: cellulose, hemicellulose, and lignin. Bioconversion of these feedstocks via the sugar route requires enzymatic hydrolysis of cellulose and hemicellulose to fermentable sugars, which subsequently are fermented to ethanol. Normally a pretreatment process is needed for efficient hydrolysis of the two carbohydrate fractions by enzymes. Typically in a pretreatment process some of the lignin is removed to increase accessibility of cellulose and hemicellulose to enzymes (U.S. Department of Energy, Breaking the Biological Barriers to Cellulosic Ethanol: A Joint Research Agenda, Report No. DOE/SC-0095 (2006)). Since the objective of lignocellulosic biomass conversion is to produce ethanol from the fermentable sugars derived from cellulose and hemicellulose, it is obvious that a good pretreatment process should preserve as much of these two carbohydrate fractions as possible.
Soaking in aqueous ammonia (SAA) has been proven to be an effective pretreatment method for lignocellulosic biomass (Kim, T. H., and Y. Y. Lee, Appl. Biochem. Biotechnol., 137-140: 81-92 (2007); Kim, T. H., and Y. Y. Lee, Appl. Biochem. Biotechnol., 124: 1119-1132 (2005)). SAA is one of the few methods for pretreatment of lignocellulosic biomass in which almost 100% glucan and about 80% xylan are retained whereas a significant percentage of lignin is removed. Despite this high efficiency in carbohydrate preservation about 20% xylan is solubilized together with lignin and hence is not available for conversion to ethanol in the subsequent fermentation step.
Thus there is a need to reduce the loss of xylan due to solubilization.